In the photolithography process for semiconductor devices or liquid crystal displays, a to-be-processed substrate is coated with a chemically amplified resist, and the resist coating film is pre-baked and exposed with a pattern, which is further subjected to post exposure baking (to be called “PEB” hereinafter), and then development. The chemical sensitization type resist is highly dependent on the environment, and if there is even a very fine amount of organic amine (for example, ammonium or N-methyl•2pyrrolidinone) or its polar molecule (these to be called “alkali component” hereinafter), a neutralization reaction occurs between the resist and the alkali component, thereby easily causing the inactivation of the resist. If the resist is inactivated, the resolution in the developing step is lowered and therefore a pattern of a desired line width cannot be obtained, causing the so-called development error. In order to avoid this, the atmosphere within the processing system, especially, the atmosphere within the PEB unit serving as a heat processing apparatus is strictly controlled so that no alkali component is present therein.
For the pattern exposure of the chemically amplified resist, a KrF excimer laser (wavelength of 248 mm) or an ArF excimer laser (wavelength of 193 nm) is employed. In the case where the KrF excimer laser is used to expose the resist film, the PEB reaction proceeds at a sensitivity of about 3 to 4 nm/° C. On the other hand, in the case where the ArF excimer laser is used to expose the resist film, the PEB reaction proceeds at a sensitivity of about 10 nm/° C. Thus, the PEB reaction is highly temperature-dependent, the heating temperature of the silicon wafer needs to be strictly managed by controlling the feeding of electricity to the hot plate with a high accuracy.
Incidentally, in the case where a currently employed recipe is changed to another recipe, the usual procedure is that the PEB process to the silicon wafer, which is currently carried out, is finished temporarily and then the temperature within the PEB unit is increased to a new set value in accordance with the new recipe. During this period, due to the radiant heat from the hot plate, the internal space of the PEB unit is gradually heated up, and at the same time, the temperature of the top cover is gradually increased. Here, if the PEB process is re-started while the temperature of the cover is being varied, several of the silicon wafers processed in the initial stage receive the radiant heat from the cover, thereby causing variation in the line width of pattern in some cases.
Jpn. Pat. Appln. KOKAI Publication No. 2002-228375 (to be referred to as Patent Document 1) discloses a PEB unit in which a heat pipe 201 is mounted in an upper section 200a of a cover 200 as shown in FIG. 1. The cover 200 has a substantially cylindrical shape and is made of aluminum or stainless steel, and includes a suction port 202 at its center. The heat pipe 201 has excellent heat conductivity and heat responsiveness, and therefore the temperature of the cover 200 quickly responds to the variation in temperature of the peripheral members, for example, the temperature variation of the hot plate. The temperature of the cover 200 is quickly stabilized by the heat pipe 201.
It should be noted that in the case of the apparatus disclosed in Patent Document 1, no matter how strictly the heating operation of the hot plate is managed, heat is radiated from the hot plate at all times, and therefore the heat is confined within the PEB unit (which is substantially an air-tight space) to create a section that is regionally over-heated. As a result, a turbulent flow is generated within the PEB unit, and the turbulent flow of hot air influences the resist coating film thermally, thereby possibly varying the line width of the pattern.
In the meantime, in some cases, the acid inhibitor (a volatile basic substance called “quencher”) contained in the chemically amplified resist is volatilized and sublimated to precipitate and attach to the periphery portion of the suction opening 202, which may be blown away by the turbulent flow to create particles of the basic material. If some of the particles of the basic substance fall onto the silicon wafer, a defect is created in the pattern circuit.
It should be noted here that in the apparatus disclosed in Patent Document 1, the temperature of the cover 200 is made to quickly respond to the radiant heat from the hot plate to stabilize the temperature within the apparatus quickly. However, this document does not at all consider the management of temperature during the PEB process. Even if the apparatus of Patent Document 1 is used during a PEB process and the temperature of the cover 200 is made to quickly follow the temperature of the hot plate to maintain the internal of the apparatus at a constant temperature, heat is radiated from the hot plate at all times, thereby confining the heat within the PEB unit (accumulation of heat) as in the above-described case and possibly creating a turbulent flow.